The present invention relates to a motor brake device for stopping an inertia-rotating rotor of a motor under an electromagnetic brake.
In a small sized tape recorder, particularly a microcassette tape recorder of which the mechanism is under a logic control, it is difficult to assemble a mechanical motor brake device into the tape recorder. In the case of a tape recorder requiring a size reduction in order to be as small as possible, such as a microcassette, the assemblage of a mechanical brake device, e.g. a motor brake mechanism, into the tape transport mechanism is accompanied by not only impeding the size-reduction of the mechanism but also increase of the production cost. A prior art device developed in light of the above problems is, for example, a brake device disclosed in Japanese Utility Model Application No. 77910/79. FIG. 1 of the present application shows a rewritten circuit diagram of FIG. 1 thereof. When the motor is forwardly rotated, transistors T1 and T2 are turned on and a drive current ID flows from one end 1 to the other end 2 of the motor M. For stopping the motor M, the transistors T1 and T2 are turned off and a transistor T3 is turned on. Upon the turning on of the transistor T3, both ends (1, 2) of the motor M are shunted via the transistor T3 and a diode D2 through which a brake current IB flows. The current IB produces and applies a magnetic field for preventing the inertia rotation of the rotor of the motor M to the inside of the motor M. Accordingly, the current IB continues its flow so long as a back electromotive force Vemf of the motor M has a voltage sufficient to flow the current IB. In accordance with the magnitude of the current IB, an electromagnetic brake is applied to the rotation of the rotor of the motor M.
FIG. 2A shows a circuit portion through which the brake current IB flows and FIG. 2B shows its equivalent circuit. For obtaining the brake current IB with a magnitude sufficient to brake the rotation of the rotor, the back electromotive force Vemf should have a magnitude larger than a given value. This will be discussed referring to FIG. 3. In FIG. 3, a curve A designates a forward voltage drop vs. forward current characteristic of the diode D2, a curve B a collector-emitter voltage vs. collector current across the collector-emitter path of the transistor T3, and a straight line C represents a resistance of an internal resistor Rm of the motor M. A voltage vs. current characteristic of the current IB loop in FIG. 2B is represented by a composite curve D of the curves A, B and C.
Assume now that the lower limit of the current IB for providing an effective brake against the rotor rotation of the motor M is IBo shown in FIG. 3. Further assume that the current lower limit IBo is 10 mA, the transistor T3 and the diode D2 are of the silicon type, and Rm=1 ohm, and that a forward voltage drop across the diode D2 is approximately 700 mV, a collector-emitter voltage drop of the transistor T3 is approximately 90 mV, and a voltage drop by the resistor Rm is 10 mV. The voltage drop caused by the diode D2, the transistor T3 and the internal resistor Rm is represented by a voltage Vemf1 at the cross point of the current lower limit IBo and the curve D. The validity of a relation IB.gtoreq.IBo requires the following relation: EQU Vemf.gtoreq.Vemf1.perspectiveto.700+90+10=800 (mV)
When Vemf&lt;Vemf1, IB&lt;IBo and the braking is ineffectively applied to the inertia rotation of the rotor.
Let us consider a case where a power supply voltage for the motor M is 1.5 V of a single manganese battery. So long as the motor M is in a forward rotation mode (or a reverse rotation mode), a current of the order of 100 mA generally flows into the motor M through the transistors T1 and T2. In this case, if the sum of the collector-emitter saturation voltages of the transistors T1 and T2 is 0.5 V, the effective supply voltage for the motor rotating in the forward mode is 1.0 V. In such a device, when the transistors T1 and T2 are turned off and the transistor T3 is turned on, the back electromotive force Vemf of the motor M is rapidly decreased from a value slightly less than 1,000 mV. As described above, if Vemf1.perspectiveto.800 mV, IB&gt;IBo is held during a period that the Vemf decreases from about 1,000 mV to about 800 mV, so that the electromagnetic damping is effectively applied to the motor M. Accordingly, until Vemf decreases down to Vemf1, the rotating speed of the motor M rapidly decreases. However, when Vemf&lt;Vemf1, the damping current IB falls below the current lower limit IBo and little braking is applied to the motor M. Therefore, in the conventional device shown in FIG. 1, when the power supply voltage for the motor M is relatively low (of the order of a few volts), the magnetic brake is ineffectively applied to the motor and a period from the interruption of the power supply to the motor M to the stop of the rotor tends to be elongated.